Stabilized control gap for spark spectroscopes



. 14, 1948. J. H. ENNS STABILIZED GONTROL GAP FOR SPARK SPECTROSCOPESFiled May 13, 1947 A T TO RN E YS Patented Dec. 14, 1948 UNITE-b STATESPArE-NroI-FICE 2,156;16

,coN'frRoL GARFOR' SPARK SPECTROSCOPES Jobo n. Enns, Ani Arbor, Mien.,assignor to Leedsl and Northrup Company, Philadelphia; Pai, acorporation ofl Pennsylvania Aii'ioat'ion May 13, 1947, serial No.747,659

iIhis invention relates to spctopy, and particularly to arrangementsfoi' 's'tbiliiing 'the light output of a spark-type spectroraphicsource.

In spark spectroscopy. an ieotno spark is used as a lightgsource toprovide as'pectim having emission lines characteristic of andidentifying thevaous elements of material used as or included in thespark electrodes iiining'- the analytical gap, Aorintroci'ucell in thegap. Espe: cially in quantitative analysis Where reproducea'nit'y andstobinty are oeeg'od, the intensity of the successive sparks must b'eheld constant Within narrow limits.

Iii accordance with the present invention, the light source isstabilized by a 'second spark gap, serving as a control gap in seriesVWith the analytical lgap, against 'whose electrodes isv directed anon-turbulent stream or streams if air or other suitable gas to removeelectrically charged particles and so insure that the breakdownpotential of the control gap remains constant, within narrow liits', forthe successive sks- The invention further resides in features ofconstruction, combination and arrangement hereinafter described andclaimed.

For more detailed understanding of the invention, reference is rn'ad tothe accompanying figures 'of the drawing's, i which:

Fig. 1 'schematically illustrates a spctrg'raph 'Simi Fig. 2, ondifferent scale, illustrates the control gap assembly, Fig. 1, embodyingthe invention;

Fig. 3 is a bottoni plan view, on reduced scale, oi an oriiice plateshown in Fig'. 2,' and Figs., 4 and 5 are explanatory iigures referredto in `discussion of" the operation of the control gap of Figs. 1 and 2.

Referring to Fig. 1 as exempla'y of a spectrogfaph system uiuiizing theinvention, iight from the analytical gap Iii is admitted into aspectrophtoniete'r represented by the blocli II in which a prisi'n orgrating produces aspec'truin having emission lines characteristic of thematerial to be analyzed, and 'for that purpose used as or in-cludedinthe ele'cI'ii-u'ies4 I2, I3 'ofthe light source lo, or ifrtroduoed intouro span; gap between them. The spectrum may be photog'raphd in Whole orin part, or it ina?, ih Whole oi' in partbe viewed by suitableradiationrespisive device, such as a bo1ret1`, photocell, or lthe like,to determine the intensity of the Isolation 'at various wavelengths, andthus duetitatively to det'erniih the amounts in which 2 various elementsare present in the composition under analysis. spectrog-raph is not hereof interest, it is not further described. It may be of any conventionaltype suitable for use in spark spectrographic analysis. I

The point here of significance is that unless the' intensity of thelight source is stable, the

measurements obtained by the spectrogaph IlA are subject to Wide unknownvariation.

The analyzing gap between the electrodes I2, I3 of the light source isincluded in a high-frequency discharge circuit including a capacitor I5,an inductor I 6, a resistor Il, and preferably a radiofrequencyamrneterl I8.

The low-'frequency vcharging circuit I9 of the capacitor I5 includes avoltage stepup transformer 20 capable of impressing high voltage, forexample of the order of 10,000 to 40,000 volts upon the capacitor I5. Toprevent undesired transfer of high-frequency energy from the dischargecircuit lll to the transformer 20, there is preferably provided anelectrostatic Shield 23 between the primary and secondary windings 2I,22, respectively, of the transformer. The lovvfifequenc'y currentexcitation for the transformer 2li` is provided by any suitable sourceof alternating current 24, such as an available 220 volt, 60 `cyclepower lin. To provide for adjustment of the charging voltage for thecapacitor I`5, there is interposed btvveen the primary Winding 22 andthe supply source 24 any suitable voltagevarying arrangement or device;preferably, and

as shown, there is utilized an adjustable auto? transformer 25 of the-"Variac type and an adjustable series resistance 26. As Willhereinafter appear, by variation of' the transformer voltage andtheprimary resistance 26, the operator may predeterniine, other thingsremaining constant, the number of sparks which jump the analytical gapper alternation of the 1oWfrequency supply voltage.

As above stated, it is essential that the light source be stable, asotherwise the spectroscopic measurements are of little value. Tostabilizethe light output of the 4analytical gap, there is included inseries with it a control gap 21 com. prising electrodes 28 and 29 ofsuitable metal; preferably tungsten, vor of Dow metal, or likemagnesium-aluminum alloys. To maintain the breakdown potential ofvthisgap constant, ltd-is keptcontinuously free of, chagedz'particles by anonturbulent stream or streams of air directed device 30.

across @ne and surfaces of the oiotroaes nomina As the particular'nature of the The oriiice plate 3I forms the upper end of a reservoir34 having volumetric capacity sufcient to reduce any turbulence arisingfrom introduction of air therein from the inlet pipe 35. The pressurewithin the reservoir 34, as read from the gage 36, may be of the orderof from ve to six centimeters of mercury above atmospheric pressure. Thedischarge openings 33 are so located that the streams of air wash theelectrode tips and clear the gap between them, particularly adjacent theelectrodes, of charged o1' ionized particles.

It is important that the air streams be nonturbulent, as otherwiseionized particles removed from the gap are randomly re-introduced, andso lower and vary through wide limits the breakdown potential betweenthe electrodes. In generallysimilar arrangements which do not providefor non-turbulent flow of air, the sparking potential between theelectrodes varied by as much as plus or minus 25 per cent, with theresult that the spectrograph readings were not reproducible fordetermination of the percentage composition of the mater-ial underanalysis in the analytical gap I0. -With the non-turbulent arrangementdisclosed, and later more specifically described, the variation inbreakdown potential of the control gap is within the limits of plus orminus two and one-half per cent, aording an accuracy satisfactory formost spectrographic analyses.

Preferably, and as shown, the jets are directed upwardly from below thegap, rather than downwardly or horizontally, so that air movement due tothe streams from the orifices 33 is in the same direction as-that due toany thermal effect of the sparks.

In the double orice arrangement shown in Fig. 2, the diameter A of eachdischarge opening 33 is of the order of the diameter of the electrodes,`which, for example, may be about one-quarter inch. To attainnon-turbulence of the air stream issuing from the discharge end of theorifice, the radius of curvature B of the approach to it is of the orderof twice the diameter of the discharge opening 33. Thus, each of thepassages 32 is dened by a surface generated by rotation of a quadranthaving the radius B about the axis ofthe passage. One radius of thequadrant is at right angles to the axis, and.

the other radius of the quadrant is parallel to the axis. Otherwiseexpressed, each passage 32 is .of circular cross section having a radiusR of magnitude, as measured at axial distance d from the dischargeopening, which may be expressed as wherein three inches. The distancebetween the axes of the passages 32 depends upon the length of the airgap between the electrodes 28 and 29. Usually the gap length is withinthe range from about one-eighth inch to three-quarters inch, and with agap length within this range, satisfactory constancy of the breakdownpotential is maintained when the center to center spacing of thepassages 32 is of the order of one-half inch. The distance between thetop of the orice plate and the vcenter line of the electrodes may bewithin the range of from about threequarters inch to yone andone-quarter inch when the electrodes and passages are of thedimensions'heren given. The reservoir 3A has sufficient capacity when,for the size orifices above discussed, the length of the reservoir is ofthe order of seven inches or so.

As the electrodes 28 and 29 are slowly consumed, it is necessary fromtime to time to readjust or replace-them. As shown in Fig. 2, `theelectrodes may be detachably carried by holders 31=31 adjustable inunison away from or toward each other by a single control knob 3Bcoupled to av gear 3.9. Electrode 29 is coupled to gear 39 by the gearstti-lll and rack 42. The holder 31 for the other electrode 28 issimilarly coupled to the gear 39 by gears MIA, IIIA and rack 42A. Bythis arrangement, the electrodes are concurrently adjustablesymmetrically with respect to the orices 33. In a common plane, theaxeseof the electrodes arein alignment with each other and at rightangles to the axes of the passages 32.

Referring vto Fig. 4, in which the sine wave E represents the opencircuit voltage across the secondary winding 2l, as the voltage rises itcharges the capacitor t5 until it attains a magnitude Eb at which aspark jumps across the control gap 2 1, whereupon the energy stored inthe capacitor I5 is dissipated as an oscillatory high-frequencydischarge in the discharge circuit I4 including the analytical gap I0.This reduces the voltage Ec across the capacitor substantiallyto zero,but as the secondary voltage is still higher, theA capacitor againstarts recharging at a rate determined -by the time constant of thecharging network and therefore the voltage Ec may again attain thepotential Eb at which a spark jumps the control gap 21. This againinitiates a high-frequency discharge, causing sparking in the analyticalgap I0. This charging and discharging cycle repeats, in each half-waveof the secondary voltage, a number of times depending upon the settingof the control gap, the time constant of the charging circuit I 9 andthe available secondary voltage of the transformer. In spectrographicwork, the number of spark breakdowns per. alternation may `range from lto 20; preferably, for most work, about threebreakdowns of the controlgap for each alternation is desirable.

For a given spacing of the control gap 21, the number of break-downs-per cycle may be in" creased by increasing the transformer secondaryvoltage available for charging the capacitor or by decreasing the valueof resistor 26. As shown by Fig. 5. when the secondary voltage E ishigher, other factors remainingthe same, the capacitor voltage Ecreaches the breakdown voltage of the control gap 2 1 a greater number oftimes per alternation because, of course, the rate at which the4capacitor 4I5 recharges is higher when the secondary voltage is higher:Alternatively, other factors remaining the same, the number of break-`downs per alternation may be increased by decreasing the value ofresistor 2S. When the breakdown potential has been predetermined by thesetting of the stabilized control gap and the number ci sparks peralternation has been set by adjustment of the transformer voltage orresistance 2B, the amount of the electrical energy dissipated per unitoi time is to a high degree constant within about five per cent,whereas, with prior arrangements the variations were of the order of tentimes as great.

The dual air stream is preferred over la single stream largely becauseof reduced consumption of air. To attain the same constancy of breakdownpotential, for a given gap setting, with a single non-turbulent streamoi air, the discharge opening must be materially increased so that itsarea is at least twice that of the combined areas of the two dischargeopenings shown in Fig. 2. However, the invention is not limited to thedual stream; a smaller or greater number of streams may be used providedthe passages are so contoured and the openings are so disposed that theair at and adjacent the tips of the electrodes is non-turbulent.

It shall be understood the invention is not limited to the specicarrangements shown, and that changes and modifications may be madewithin the scope of the appended claims.

What is claimed is:

l. A control spark gap for spectrographic analysis comprising spacedsparking electrodes, and structure having a passage contoured tominimize turbulence in a stream of gas directed across the spark pathbetween the tips oi' the electrodes to remove ionized particles fromsaid path in the interval between successive sparks.

2. A control spark gap for spectrographic analysis comprising spacedsparking electrodes, and structure having a circular orice with a radiusof curvature of the approach to the discharge opening of the orifice ofthe order of twice the diameter of said opening for directing air acrossthe spark path and against at least one of the electrodes to removeionized particles from said path in the interval between successivesparks.

3. A control spark gap for spectrographic analysis comprising spacedsparking electrodes and an orifice plate structure having passagestherethrough whose axes are parallel and spaced respectively to directstreams of air across the tips of the electrodes, the diameters of thedischarge openings of said passages being of the order of the diameterof the electrodes and the cross sectional areas of said passages sodecreasing towards their respective discharge openings that the airstreams issuing therefrom are non-turbulent.

4. A control spark gap for spectrographic analysis comprising sparkingelectrodes and circular plate structure having a diameter at least tentimes the electrode diameter and through which extend passages Whoseaxes are parallel and so spaced that air streams therefrom respectivelyclear the tips of the electrodes and the adjacent regions oi chargedparticles, the diameter oi the discharge end of each passage being ofthe order ci the electrode diameter and the cross sectional of eachpassage so decreasing towards its discharge end that the air streamissuing therefrom is non-turbulent.

5. A control spark gap for spectrographic analysis comprising spacedsparking electrodes, and structure having a passage for directing airagainst the tips of said electrodes, said passage decreasing in crosssectional area in direction of flow of the air therethrough and whoseradius (R) at each distance (d) from the discharge opening of thepassage may be expressed as wherein A :diameter of discharge opening 0sin1 6. The method of obtaining constancy of the potential at whichsuccessive sparks will form between the electrodes of the control gapfor spectrographic analysis which comprises continuously directingacross the electrode tips a nonturbulent stream of air to insure removalof charged particles from the control gap.

7. The method of predetermining the mean intensity of the light outputof a spark source which comprises the step of selecting the peakmagnitude for an alternating Voltage used to charge a reactance,selecting the time constant of the charging circuit including saidreactance, selecting the length of a control gap in circuit with saidsource and said reactance to predetermine its breakdown voltage fordisruptive discharge of said reactance, and directing non-turbulent airacross said control gap to maintain the breakdown voltage constant forsuccessive sparks, said steps jointly determining the number and energycontent of the sparks per alternation of the charging voltage.

JOHN H. ENNS.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Number Name Date 1,638,336 Himes Aug. 9, 19271,711,983 Bassett May '7, 1929

